Method and device for measuring emissions of gaseous substances to the atmosphere using scattered sunligt spectroscopy

ABSTRACT

Methods for measuring emissions of gaseous substances to the atmosphere using scattered sunlight spectroscopy and an optical measuring device are disclosed in which the device includes a telescopic member defining a field-of-view of the optical measuring device and a scanner for controlling variation of the direction of the field of view to scan a predetermined layer of the atmosphere, the method comprising scanning the field-of-view to scan the predetermined layer of the atmosphere in the form of at least a part of a cone having its apex positioned at the optical measuring device and having a cone angle β. Optical measuring devices themselves are disclosed.

FIELD OF THE INVENTION

The present invention relates to a method for measuring emissions ofgaseous substances to the atmosphere using scattered sunlightspectroscopy and an optical measuring device. The present invention alsorelates to an optical measuring device for use in such a method.

BACKGROUND OF THE INVENTION

Gaseous substances are constantly emitted to the atmosphere from bothanthropogenic sources, such as vehicles and industries, and naturalsources, such as volcanoes. Many of these substances have an effect onhuman health and/or on the global climate, the protective ozone layer inthe stratosphere or other environmental effects. In order to study sucheffects and to quantify different sources etc., it is important thatthere are suitable equipment and methods available for measuringemissions and flux of such gaseous substances. The principles of somedifferent methods are described in e.g. WO 02/04902.

Measurement of gases from volcanoes are important not only for studyingtheir environmental impact and their effect on atmospheric chemistry,but also with regard to geo-science, as the gases emitted from a volcanocarry important information on the geophysical and geochemical status ofthe interior of the volcano, as well as to risk mitigation/assessment,since the gas emission and composition adds to our understanding of thevolcanoes status.

In measurements of gases emitted from relatively well demarcatedsources, such as discharges from industries and volcanoes, the emissionscan be measured at, or at least relatively close to, the source. In suchmeasurements methods based on scattered sunlight spectroscopy areuseful. In principle, scattered sunlight spectroscopy involves the useof a spectrometer to record the light of the zenith sky, i.e. the skystraight above the spectrometer, wherein an integration is made over thewhole of the cross section of the emitted air mass. This results in aspectrum of the zenith sky including the vertically integratedconcentration of the gases which are present in the atmosphere. Bymoving the spectrometer in such a way that the vertical layer that ismeasured cuts the emitted gas mass, e.g. an emission plume from asource, it is possible, after geometrical correction for deviations indirection of movement from a direction perpendicular to the propagationof the gas mass and after correcting for background concentration, todetermine the integrated concentration over a cross section of theemitted gas mass. The emission is obtained after multiplying thisintegrated concentration with the concentration-weighed wind speed

A scattered sunlight spectroscopy instrument that has been usedextensively in studies of volcano emissions is the so-calledCOSPEC-instrument, which is an optical remote sensing instrumentdeveloped in the 70's. It consists of a spectrometer that receivesscattered UV-light from a narrow solid angle of the blue sky, typicallyat zenith. SO₂ has a characteristic absorption spectrum around 300 nm.By means of a mask correlation technique the instrument is able toderive the total column of SO₂ molecules along the vertical path orcolumn defined by the instruments field-of-view (FOV). Thus thevertically integrated number of SO₂ molecules in a segment of a volcanicplume may be derived. By installing the instrument on a moving platform,and move the platform in a direction such that the instruments FOV cutsthe volcanic gas plume, the total number of SO₂ molecules in across-section perpendicular to the direction of propagation of the plumemay be derived. Multiplying this number with the molecular weight andthe wind-speed gives the mass-flux in kg/s. The COSPEC-instrument hasthe advantage that the measurements can be made remotely at distances ofseveral km from the source and that mass flux can be derived. For thesereasons it has become the most important technique for volcanic gasemission monitoring.

Technological development during the recent decades has resulted insensitive and fast multi-channel array detectors, powerful computers,and algorithms for modelling of radiative transfer and accurate analysisof differential absorption spectra. This has led to an alternative tothe COSPEC-instrument: a miniature fiber optic ultraviolet differentialoptical absorption spectrometer: the mini-DOAS, which is described inGalle B., Oppenheimer C., Geyer A., McGonigle A., Edmonds M. andHorrocks L., “A miniaturised ultraviolet spectrometer for remote sensingof SO₂ fluxes: a new tool for volcano surveillance”. Journal ofVolcanology and Geothermal Research, Volume 119, Issues 1-4, 1 Jan.2003, Pages 241-254. The basic mini-DOAS system consists of a pointingtelescope fiber-coupled to a miniature spectrograph. Ultraviolet lightfrom the sun, scattered from aerosols and molecules in the atmosphere,is collected by means of the telescope (length 10 cm, diameter 3 cm)with a quartz lens defining a field-of-view of 8 mrad. Light istransferred from the telescope to the spectrometer by means of a 2 mlong optical quartz fiber with 800 μm diameter. The spectrometer uses a2400 lines/mm grating combined with a 50 μm slit, providing an opticalresolution of ˜0.6 nm over a wavelength range of 245-380 nm. Thesoftware used for controlling the spectrometer as well as for evaluatingthe spectra is a custom-built program.

In a typical zenith sky measurement the mini-DOAS is carried by car orfoot, and spectra are recorded whilst the instrument is moved under thevolcanic gas plume in a direction approximately perpendicular to theplume transport direction. The platform position is tracked using a GPSreceiver making it possible to geometrically correct for deviations inmovement from the ideal direction perpendicular to the plume transportdirection. In this manner the total number of molecules in a thin layerof the plume can be determined, and after multiplication with wind speedat plume height, the mass flow of the emission can be obtained, e.g. inkg/s. Besides being a cost-effective alternative to the COSPEC, theintroduction of the mini-DOAS represented a major step forward in termsof field-operability, mobility and flexibility. The small size and powerconsumption of the device made possible measurements in hithertoinaccessible areas and situations.

The mini-DOAS has been further developed in that the instrument wascoupled to a scanning device consisting of a quartz-prism attached to acomputer-controlled stepper-motor, providing a means to scan thefield-of view of the instrument over 180°, see Edmonds M., R. A. Herd,B. Galle and C. Oppenheimer, “Automated, high time-resolutionmeasurements of SO₂ flux at Soufrière Hills Volcano, Montserrat”,Bulletin of Volcanology, Vol 65, 578-586, 2003. In a typical measurementthe instrument is located under the plume, and scans are made, fromhorizon to horizon, in a plane perpendicular to the wind-direction.Thus, instead of moving the measurement equipment under the gas plume,this version of the mini-DOAS is capable of scanning a plane in the skyfrom a stationary position. Of course, this feature simplifies themeasurement considerably. A stepwise scan of the plane typically use a10 seconds integration time with 7.5° angular resolution, providing afull emission measurement every 4 minutes. The scanning mini-DOASprovides time resolved measurement of the gas emission, making itpossible to correlate gas emission data with other geophysical data,e.g. seismic signals. A similar approach but using a mirror instead of aprism in the scanning device has also been used for volcanic gasmeasurements, McGonigle A. J. S., C. Oppenheimer, A. R. Hayes, B. Galle,M. Edmonds, T. Caltabiano, G. Salerno and M. Burton, “Volcanic sulphurdioxide flux measurements at Etna, Vulcano and Stromboli obtained usingan automated scanning ultraviolet spectrometer”, Journal of GeophysicalResearch, vol 108, No B9, 2455, 2003.

As mentioned above one of the main advantages of the scanning mini-DOASis that it is a stationary instrument that does not have to be movedduring the measurement. However, this also means that the emission canonly be measured when the wind direction is such that the gas plume fromthe volcano is passing over the instrument within a certain angularinterval around zenith. If the wind direction changes such that it fallsoutside this interval the gas plume will, at least partly, beintersected by the instruments field-of-view at too high zenith anglesto facilitate a good measurement. Typically it will not be able todetermine unambiguously the clean air background below the plume. Aneffect of this is that it becomes difficult to evaluate the integratedconcentration in the gas plume which leads to an unreliabledetermination of the amounts of emissions in the plume. In order tosolve this problem efforts have been made to improve the mathematicalalgorithms for calculating the area of such incomplete plume scans byassuming certain plume dispersion characteristics and applyingmeteorological dispersion models. Another, more practical way, to solvethis problem is to move the instrument or to install a number ofinstruments around the emission source such that the measurement rangesof two adjacent instruments overlap.

Another problem related to this measurement strategy is caused byscattering of light in the lower part of the atmosphere. In thedescribed measurement strategy it is assumed that all the light thatreach the instrument originates from a volume above the gas plume to bestudied. If the Sun is located at an angle off zenith, direct light fromthe Sun may be scattered by aerosols and molecules located between theinstrument and the gas plume, into the field-of-view of the instrument.This part of the recorded light has never passed the gas plume and thuscauses a “diluted” spectrum, giving rise to an underestimation of thegas content in the plume. This problem gets worse the more aerosols(haze, particles, ash, rain, etc.) there is in the lower atmosphere. Italso gets worse when the plume is intersected at high zenith angles(close to the horizon) and when the distance between the instrument andthe gas plume is long.

To calculate the emission using the above mentioned methods, a knowledgeof the transport speed of the gas plume is necessary. This can beobtained either by meteorological modeling, by extrapolatingmeteorological measurements made elsewhere or by measurement. A methodfor measuring the transport speed of the gas plume directly has beendeveloped using DOAS techniques. In this method two DOAS instruments,similar to the systems described above, are located under the gas plumeand made to point in two different directions, one beam upwind and theother downwind the plume, along the plume propagation direction. A timeseries of total column variations are registered in both directions, andfrom the temporal delay in variations in the total column, thewind-speed can be calculated if the plume height is known. Instead oftwo separate DOAS instruments one single system may be used where aflipping mirror instead alternates the pointing direction of the systembetween two directions. Ideally the parameter to be measured is the windspeed at plume height, weighted by plume concentration. As this approachuses variations in the total column of SO₂ to determine the plume speed,a weighting with concentration is automatically made. Examples of windmeasurements using this approach at Etna Volcano on Sicily andPopocatepetl Volcano in Mexico are given in Galle B., M. Johansson, C.Rivera and Y. Zhang, “Dual-Beam mini-DOAS spectroscopy, a novel approachfor volcanic gas emission monitoring”, EGU General Assembly, Vienna,24-29 Apr. 2005.

One object of the present invention is to provide a method and a devicefor measurements of the type described above, which method and deviceexhibit improved properties compared to conventional instruments andmethods, in particular with regard to changing wind directions.

SUMMARY OF THE INVENTION

In accordance with the present invention, this and other objects havenow been realized by the discovery of a method for measuring emissionsof gaseous substances to the atmosphere using scattered sunlightspectroscopy and an optical measuring device comprising a telescopicmember defining a field-of-view of the optical measuring device, and ascanner for controlling the variation of the direction of thefield-of-view to scan a predetermined layer of the atmosphere, themethod comprising scanning the field-of-view to scan the predeterminedlayer of the atmosphere in the form of at least a part of a cone havingits apex positioned at the optical measuring device and having a coneangle β. Preferably, the cone angle β is up to 80°, more preferably atleast 20°. In a preferred embodiment, the angle β is from 40 to 70°.

In accordance with another embodiment of the method of the presentinvention, the cone has its axis of symmetry in a horizontal direction.In another embodiment, the cone has an axis of symmetry exhibiting aninclination angle (δ) relative to a horizontal plane. Preferably, theinclination angle (δ) is 90°−β.

In accordance with the present invention, this and other objects havenow been overcome by the discovery of an optical measuring device formeasuring emissions of gaseous substances to the atmosphere usingscattered sunlight spectroscopy, the device comprising a telescopicmember defining a field-of-view of the measuring device, and a scannerfor controlling the variation of the direction of the field-of-view toscan a predetermined layer of the atmosphere, the scanner being adaptedto scan the predetermined layer of the atmosphere in the form of atleast a part of a cone having its apex positioned at the opticalmeasuring device and having a cone angle β. Preferably, the cone angle βis up to 80°, and in another preferred embodiment, the cone angle β isat least 20°. In a preferred embodiment, the cone angle β is from 40 to70°. In accordance with another embodiment of the device of the presentinvention, the scanner comprises a light refracting member arranged todeflect the field-of-view by directing a portion of incoming sunlighttowards the telescopic member. Preferably, the light refracting memberis arranged in such a way that the non-deflected field-of-view and thedeflected field-of-view form the angle β. In another embodiment, thelight refracting member is rotatably arranged in relation to thetelescopic member whereby rotation of the light refracting member allowsthe field-of-view to scan the predetermined layer of the atmosphere. Ina preferred embodiment, the device includes a motor having a motor axis,and the light refracting member is arranged on the motor axis connectedto the motor. In another embodiment, the light refracting member is amirror. In yet another embodiment, the light refracting member is aprism.

In accordance with another embodiment of the device of the presentinvention, the device includes a light analysis unit optically connectedto the telescopic member. Preferably, the light analysis unit comprisesa spectrometer.

In accordance with another embodiment of the device of the presentinvention, the telescopic member comprises a light focusing memberadapted to focus light that enters the telescopic member and an openingadapted to direct the focused light out from the telescopic member.Preferably, the light focusing member is a lens. In another embodiment,the light focusing member is a mirror.

The present invention concerns a method for measuring emissions ofgaseous substances to the atmosphere using scattered sunlightspectroscopy and an optical measuring device, said device comprising atelescopic member defining a field-of-view (FOV) of the device, and ascanning arrangement allowing a controlled variation of the direction ofthe field-of-view (FOV) in such a way that the field-of-view (FOV) iscapable of scanning a certain layer (12, 22) of the atmosphere duringoperation of the device. The inventive method is characterized in thatit comprises the step of scanning a scanning layer that has the form of,at least a part of, a cone having its apex positioned at the device andhaving a cone angle β.

Such a scanning layer is capable of being directed towards an emissionsource and partly surrounding the source resulting in severaladvantageous effects. One advantage is that it increases the winddirection interval within which a reliable measurement can be performed.A consequence of this is that a wider range of wind directions can becovered by a single instrument, or alternatively that a smaller numberof stationary measuring devices are required to cover all possible winddirections around an emission source. Another advantage is that thedistance between the measuring device and the location where an emissionplume is scanned is decreased which improves the reliability of themeasurement as the previously described scattering effects are reduced.This is of particular importance when the wind direction deviatesconsiderably from a direction right above the place where the measuringdevice is positioned.

In a preferred embodiment of the inventive method, the cone angle β is≦80° and ≧20°. Preferably, the cone angle β is in the interval 40-70°.

The invention also concerns an optical measuring device for measuringemissions of gaseous substances to the atmosphere using scatteredsunlight spectroscopy, said device comprising a telescopic memberdefining a field-of-view (FOV) of the device, and a scanning arrangementallowing a controlled variation of the direction of the field-of-view(FOV) in such a way that the field-of-view (FOV) is capable of scanninga certain layer of the atmosphere during operation of the device. In theinventive device, the scanning arrangement is adapted to form a scanninglayer that has the form of, at least a part of, a cone having its apexpositioned at the device and having a cone angle β.

In a preferred embodiment of the inventive device, the scanningarrangement comprises a light refracting member arranged to deflect thefield-of-view (FOV) by directing a portion of incoming sunlight towardsthe telescopic member. Such a design makes it possible to control thelight refracting member, instead of the telescopic member, as to varythe direction of the FOV, which is less complicated.

Preferably, the light refracting member is arranged in such a way thatthe non-deflected FOV and the deflected FOV forms the angle β. Nofurther parts are thus needed for achieving this angle.

In a further preferred embodiment of the inventive device, the lightrefracting member is rotatably arranged in relation to the telescopicmember in such a way that rotation of the light refracting member allowsthe field-of-view (FOV) to scan the scanning layer. Preferably, thelight refracting member is arranged on a motor axis connected to amotor.

BRIEF DESCRIPTION OF THE DRAWINGS

In the description of the present invention set forth in the followingdetailed description, reference is made to the following figures, inwhich:

FIG. 1 is a front, perspective, schematic view of a principalillustration of the use of a prior art optical measuring device,

FIG. 2 is a front, perspective, schematic view of a principalillustration of the use of the method and device of the presentinvention,

FIG. 3 is a graphical representation of an example of the measurementresults obtained with the prior art device according to FIG. 1,

FIG. 4 is a graphical representation of an example of the measurementresults obtained with the inventive method and device according to FIG.2,

FIG. 5 is a schematic, representational view of an example of how thepresent invention can be used to surround an emission source,

FIG. 6 is a top, schematic, representational view of the example of FIG.5 as seen from above, together with a similar view of a prior artdevice,

FIG. 7 shows another example of the use of the invention,

FIG. 8 is a schematic, representational view of the use of four priorart devices arranged around an emission source,

FIG. 9 is a schematic, representational view of the use of four devicesof the present invention arranged around an emission source,

FIG. 10 is a top, elevational, schematic view of a preferred embodimentof the optical measuring device of the present invention, and

FIG. 11 is a graphical representation of the present invention actualmeasurement results obtained with a prior art device and with the deviceof the present invention.

DETAILED DESCRIPTION

FIGS. 1 and 2 show a 3-d representation of the measurement strategiesfor the prior art and the present invention, respectively. FIG. 1 givesa principal illustration of the use of a prior art optical measuringdevice 10 in the form of a scanning mini-DOAS in measurements of a gasemission from a source in the form of a volcano 5. In particular, FIG. 1illustrates the effect of different plume directions relative to thelocation of the measuring device 10. Three different plumes P1, P2 andP3 represent three different wind directions W1, W2 and W3. The firstwind direction W1 is directed from the gas emission source 5 towards themeasuring device 10 resulting in that the corresponding plume P1 passesright above the measuring device 10. The second wind direction W2, andthus the direction of the second plume P2, exhibit an angle Φ₁ relativeto the direction of W1 and P1 such that the second plume P2 passes abovethe measuring device 10 somewhat at the side. The third wind directionW3, and thus the direction of the third plume P3, exhibit an angle Φ₁+Φ₂relative to the direction of W1 and P1 such that the third plume P3passes above the measuring device 10 further at the side. FIG. 1 furthershows a scanning layer 12 which in this case has the form of a verticalplane, and which will be further described below.

As described further above the prior art scanning mini-DOAS scans thefield-of view (FOV) of the instrument step by step from horizon tohorizon. In each scanning position, i.e. in each step, the FOV defines amore or less slanting column extending through the atmosphere whichcolumns contain the air to be measured. A number of such columns areindicated by lines 14 in FIG. 1. Each of these lines 14 represents asolid angle defined by the FOV of the instrument. For clarity reasonsthese lines 14 have been given a certain length although the columns orFOV's strictly speaking have an infinite length. All the columns 14taken together define the scanning layer 12 which in FIG. 1 has the formof a vertical plane. Indicated in FIG. 1 is also a scan step angle α.When the FOV is in a zenithal position, i.e. when it is directedvertically α is 0° and when it is directed horizontally α is either 90°or −90°. FIG. 1 further shows sectional views of the plumes P1, P2 andP3 taken along the scanning layer 12.

FIG. 2 shows the principles of the present invention in a situationsimilar to what is shown for the conventional equipment in FIG. 1. Inthe example shown in FIG. 2 the inventive measuring device 20 isarranged such that in each scanning position the FOV defines a column(indicated by lines 24) that exhibits an angle β of 45° relative to adirection defined by an imaginary, horizontal line 21 drawn between themeasuring device 20 and the emission source 5. This results in a curvedscanning layer 22 that has the shape of an upper half of a horizontalcone having its axis of symmetry 21 corresponding to the imaginary line21, having its apex positioned at the measuring device 20 and having acone angle β=45°. This means that the cone, and thus the scanning layer22, has an opening angle in the horizontal plane, i.e. the angle betweenthe lowest, substantially horizontal, columns 24′, that is 2·β=90° and avertical opening angle, i.e. the angle between the most vertical column24″ and ground, that is β=45°. The scan step angle a has the samemeaning in FIG. 2 as in FIG. 1; when the FOV is in its most verticalposition, i.e. when it is directed as indicated by line 24″, a is 0° andwhen it is directed horizontally, i.e. when it is directed as indicatedby lines 24′, α is either 90° or −90°. FIG. 2 further shows sectionalviews of the plumes P1, P2 and P3 taken along the scanning layer 22.

In contrast to the planar scanning layer 12 shown in FIG. 1, thescanning layer 22 of the present invention exemplified in FIG. 2 has anopening angle in the horizontal plane that is less than 180° (which itwill have as long as the cone angle β is less then 90°). For this reasonthe scanning layer 22 becomes funnel-shaped which make it possible todirect the scanning layer 22 towards the emission source 5 and therebyletting the scanning layer 22 at least partly surround the source 5.This has some advantages which will be described further below.

FIG. 3 shows a principal absorption column representation of a derivedcontent of a gaseous substance, in this example SO₂, in a particulardirection of the FOV, i.e. in a particular air column, as a function ofscan step angle α obtained with a prior art measuring device 10 in ameasurement corresponding to the conditions of FIG. 1. As plume P1passes straight above the measuring device 10 at α=0° the maximum slantcolumn for P1 is positioned in the middle of the scan with backgroundvalues on each side of the peak resulting in that a good measurement ofthe integrated mass in plume P1 can be performed. Regarding plume P2 areasonably good measurement can still be performed as a backgroundmeasurement is obtained below the plume at about −80°. Regarding plumeP3 no reliable measurement can be performed as only a part of the plumepeak is registered, i.e. the FOV is still directed towards the plume atthe lowest scan angle which in this example is −80°. A change of thewind direction from W1 to W3, i.e. a change of Φ₁+Φ₂ degrees, thus makesthe measurement unreliable. Considering the peak of P2 in FIG. 3 to besufficiently defined for an acceptable reliability of the measurementthe acceptable change in wind direction for a reliable measurement withthe prior art mini-DOAS is thus +/−Φ₁ if the measuring device 10initially is placed right below the plume.

As a comparison to FIG. 3, FIG. 4 shows a similar principal absorptioncolumn representation as shown in FIG. 3 but obtained with an inventivemeasuring device 20 in a measurement corresponding to the conditions ofFIG. 2. Plume P1 passes straight towards and above the inventivemeasuring device 20 such that at α=0° the maximum slant column for P1is, in similarity with FIG. 3, positioned in the middle of the scan withbackground values on each side of the peak resulting in that a goodmeasurement of the integrated mass in plume P1 can be performed.Regarding plume P2 a good measurement can be performed as a backgroundmeasurement is obtained below the plume at about −60°. In contrast toFIG. 3 where only a part of the plume peak of P3 is registered, in thiscase also plume P3 can be reasonably well measured as a backgroundmeasurement is obtained below the plume at about −80°. Thus, a change ofthe wind direction from W1 to W3, i.e. a change of Φ₁+Φ₂ degrees, doesnot make the measurement unreliable. Considering the peak of P3 in FIG.4 to be sufficiently defined for an acceptable reliability of themeasurement the acceptable change in wind direction for a reliablemeasurement with the inventive measuring device 20 is thus +/−(Φ₁+Φ₂) ifthe measuring device 20 initially is placed right below the plume.

Due to topographical and atmospheric radiation limitations, there isnormally a lower limit in scan angle (α) below which no reliable slantcolumn density can be derived. Under good conditions this angle istypically 80°-85°.

As mentioned above, the inventive scanning layer 22 is funnel-shapedwhich makes it capable of partly surrounding the source 5. This leads toat least two major advantages. Firstly, it increases the wind directioninterval within which a reliable measurement can be performed. This isillustrated in FIGS. 2 and 4, as compared to FIGS. 1 and 3, showing thatalso the third plume P3 now can be measured reliably since backgrounddata can be obtained also for scan step angles α below the plume P3.This is a major improvement compared to the measurement with theconventional equipment. A consequence of this is that a wider range ofwind directions can be covered by a single instrument, or alternativelythat a smaller number of stationary measuring devices 20 are required tocover all possible wind directions around an emission source 5.Secondly, the distance between the measuring device 20 and the locationwhere the plume is scanned is decreased which improves the reliabilityof the measurement as the previously described scattering effects arereduced. This is illustrated in table 1 where the distance betweeninstrument and gas plume is shown as a function of wind directiondeviation Φ for the two different measurement strategies, using as anexample a typical volcano with a height of 1 km, located 3 km away andthe instrument having a cone angle β=45°.

Table 1 shows that the distance from the measuring device to the gasplume, for wind direction deviations Φ of more than around 30°, isconsiderably shortened with the scanning layer 22 according to theinvention compared to the scanning layer 12 according to conventionaltechnique. It should be noted that for Φ>80°, the conventional techniquestarts to get useless, while the inventive technique still give usefuldata even beyond Φ>100°, thus making possible the total surrounding of asource using only two measuring systems (see FIG. 5). For wind directiondeviations Φ of less than around 30°, corresponding to the plume centerpassing close to zenith, the present invention gives a slightly longerdistance between instrument and plume. However, this distance is herestill relatively short so this minor disadvantage is well compensated bythe strong advantage at larger, more critical, wind directiondeviations.

TABLE 1 Distance from instrument to the point where the measurementintersects the gas plume, for different plume direction deviations.Plume height 1 km, distance to volcano 3 km, β = 45°. ConventionalInvention Wind direction geometry geometry deviation Φ scan layer 12scan layer 22 (degrees) (km) (km) 0 1.00 1.41 10 1.13 1.50 20 1.48 1.6930 2.00 1.94 40 2.72 2.21 50 3.72 2.51 60 5.34 2.83 70 8.32 3.20 8017.37 3.65 90 ∞ 4.26 100 N/A 5.13 110 N/A 6.59

Another reason for the improvement of the measurement reliability withthe new scanning geometry is that, ideally for the flux measurement, andeven more important for the plume-speed measurement, the scanning shouldbe made in a plane relatively close to perpendicular to the plumepropagation. In table 2 is shown the angle between the horizontalprojection of the direction of the instruments FOV and the plumepropagation direction, for different plume propagation directions Φ. Asthe wind direction is deviating more and more from 0, the angle underwhich the plume is scanned is deviating more and more from perpendicularto the plume propagation. With the inventive scanning layer 22 thiseffect is strongly suppressed. For wind direction deviations 0 of lessthan around 60°, the deviation from the ideal perpendicular traverse isstill less than 20°, and even for Φ=90° the deviation is not more than47° with the inventive device.

TABLE 2 Angle between the horizontal projection of the direction of theinstruments FOV and the plume propagation direction, for different plumedirection deviations. Plume height 1 km, distance to volcano 3 km, β =45°. Conventional Invention Wind direction geometry geometry deviation Φscan layer 12 scan layer 22 (degrees) (degrees) (degrees) 0 90 90 10 10082 20 110 81 30 120 86 40 130 93 50 140 100 60 150 109 70 160 118 80 170127 90 N/A 137 100 N/A 146 110 N/A 155

In principal, these improvements gradually increase as the cone angle βis reduced from 90°. However, the effect is not likely to be significantwhen β>80°. For geometrical reasons it is likely that the error in theflux calculations increases when the cone angle β gets too small.Nevertheless, as a low value of β makes it possible to completelysurround the source with fewer instruments, low values of β, less than45°, may still be attractive in some applications where a detection of adramatic change in emission is interesting, e.g. the opening of aconduit system of a volcano. An example of a surrounding arrangement isgiven in FIG. 5. Here two systems are set up around a volcano at adistance of 2 km. The volcano is 1 km high. The cone angle β was 45° andthe scan angle α was ranging between ±70° with 5° steps. As can be seenthe two scanning systems puts up an effective “grid” facilitating thedetection of gas emission in all possible directions, providing a costeffective way to detect dramatic changes in the gas emission.

The lower part of FIG. 6 shows a top view of FIG. 5 where theintersection between the FOV columns 24, i.e. the conical scanning layer22, and the plume plane at 1 km height are indicated with crosses. As acomparison, the upper part of FIG. 6 also shows a top view of twoconventional measuring devices 10 located in the same positions as theinventive devices in FIG. 5 and lower part of FIG. 6. It is clear thatthe two parallel, planar, vertical scanning layers 12 formed by theconventional devices do not cover varying wind directions from thevolcano 5 as well as the inventive devices.

In a variant of the inventive cone-shaped scanning layer 22 describedabove the axis of symmetry of the cone is no longer horizontal butinstead have an inclination δ. A special case of this embodiment is whenδ=90°−β. Then the scanning direction will point towards zenith when α=0.This may be advantageous in some applications, e.g. if the earlierdescribed dual-beam method is used for plume speed measurements. Afurther special case of this embodiment is if the cone inclination δequals the inclination angle from the instrument to the source. In thisspecial case the cone axis point towards the source. This has theadditional advantage that the scan makes an almost vertical intersectionof the plume when α is around ±90°. It also makes the scan perpendicularto the plume for low scan angles α. This reduces the error in the fluxcalculation caused by geometrical inaccuracies as well as improves thepossibility to make successful measurements using the dual-beam plumespeed measurement approach. An example of such a special embodiment setup for measurements of emission from a volcano using two measurementsystems is shown in FIG. 7. Here ∂=90°−β=30°=inclination angle betweeninstrument and volcano.

Further advantages of the inventive scanning layer are that it improvesthe possibility to make 2-d representations of the concentration fieldof the gas plume, using two or more measuring devices. For calculationsof the flux from the data obtained with the instrument a knowledge ofthe height of the gas plume, or ideally the concentration distributionin the plane of the scanning, is necessary. This is needed for threereasons; Firstly, because the wind speed usually shows a strong gradientwith height. Thus knowledge of the plume height is important in order tobe able to establish the correct plume speed to calculate themass-transport. Secondly, when calculating the total number of moleculesin a cross-section of the plume each scanning direction represents acertain scanned area along the cone surface. This area increaseslinearly with the distance from the instrument. Thus, information aboutthe plume height, and ideally also the distribution of the gasconcentrations along the scanned cone surface, is important to obtain acorrect number of molecules integrated over the scanned area of theplume. Finally information of the plume height is also necessary if theearlier described dual-beam method to derive plume speed is used.Information about the geographical distribution of the gas over thescanned area can be obtained from modelling, e.g. calculation of theexpected plume lift, and using an appropriate dispersion model.Alternatively the distribution of the gas concentration over the scannedcone surface may be measured using two or more instruments scanning theplume from different directions, and making a tomographic reconstructionof the concentration field. This technique is relatively straightforwardwhen the two systems are scanning in the same plane. When severalsystems are installed around a source a major reason for doing so is,however, to be able to cover varying wind directions. To maximize thecoverage of different wind directions a more optimal setup is todistribute the measuring devices around the source. A disadvantage withsuch a setup when using the conventional technique is that the twosystems now no longer scan the plume in the same geographical plane, anda tomographic reconstruction of the concentration field gets morecomplicated and less accurate. With the cone scanning method, choosingappropriate scanning parameters, a higher degree of overlap may beobtained, while also improving the coverage of varying wind directions.It can be shown that at the altitude where the plume intersection isexpected, the overlap between the scans from the two instruments isalmost perfect. A more extreme demonstration of this advantage,surrounding a volcano with four systems, is shown in FIGS. 8 and 9. Herethe four systems in the conventional configuration (see FIG. 8) arescanning orthogonal to each other, with no overlap at all, while theinventive configuration (see FIG. 9) still shows an almost perfectoverlap.

FIG. 10 shows a preferred embodiment of an optical measuring device 20according to the invention. The device 20 comprises a telescopic member30 provided with a lens 39 and a focal point 40 at which an opticalfiber 33 is attached as to optically connect the telescopic member 30 toa spectrometer 31 via its entrance slit 41. The telescopic member 30defines an optical axis 38 and a field-of-view (FOV) 24 directed towardsa scanning arrangement 42. The scanning arrangement 42 comprises acomputer-controlled stepper-motor 36 that, via a motor axis 35, isconnected to a mirror 32 mounted onto a supporting member 34, whereinthe mirror 32 faces the telescopic member 30 as to establish an opticalconnection. The mirror 32 is arranged so that the direction of the FOV24 forms an angle β with the optical axis 38. A certain portion ofscattered sunlight 37 falling within the FOV 24 into the mirror 32 willthus be directed towards the telescopic member 30 and thus to thespectrometer 31. When the motor axis 35, and thus the mirror 32, isrotated the FOV 24 of the telescopic member 30 defines the scanninglayer 22 (see FIG. 2) in the atmosphere which layer 22 forms a part of acone with an axis of symmetry along the optical axis of the telescopicmember 30, a top at the mirror 32 and a cone angle β.

If the measuring device 20 is used in a horizontal direction, i.e. ifthe optical axis 38 and the co-axially arranged motor axis 35 arehorizontal, the symmetry axis of the scanning cone will also behorizontal. As mentioned above, the measuring device 20 can however betilted vertically with an angle δ, resulting in that the symmetry axisof the scanning cone gets a vertical inclination δ.

From geometrical laws it follows that the mirror 32 exhibits an angle γwith a plane 44 perpendicular to the motor axis 35 and the optical axis38 where γ=β/2. It also follows that the mirror 32 exhibits an anglewith the axes 35, 38 that is equal to (90−γ)°. In this example β=45°which means that γ=22.5°.

An instrument based on the above mentioned ideas has been built and itsperformance was tested in a field test on the gas-plume from theactive'volcano Popocatepetl in Mexico. The measuring device 20 used inthe field test consists of a small telescope 30 (quartz lens, focallength=50 mm, diameter=20 mm, field of view approximately 0.4°) coupledto a quartz fiber bundle 33, which transmits the light 37 into acommercial miniature fiber optic spectrometer 31 (OceanOptics Inc.,USB2000). The telescope 30 is optically connected to a scanning device42 consisting of a mirror 32 attached to a computer-controlledstepper-motor 36, providing a means to scan the field-of view 24 of theinstrument over 180°, i.e. the motor axis is rotated 180°. The mirror 32was positioned such that an angle β was formed with the optical axis 38as shown in FIG. 10 as to form a cone shaped scanning layer 22. In thisparticular experiment the angle β was 45°. Typically a secondsintegration time was used, with 7.5° angular resolution, providing afull emission measurement every 4 minutes. The 2 m long quartz fiberbundle 33 consisted of four individual 200 μm core diameter fibers,arranged in rectangular configuration in the focal point 40 of thetelescope 30 and a linear arrangement at the spectrometer entrance slit41. A Hoya U330 filter blocked the visible light >400 nm to reduce thestray light in the spectrometer. The USB2000 spectrometer 31 used was acrossed Czerny-Turner arrangement (1/f=2.2, UV grating 2400 grooves/mm)with a CCD detector (2048 elements at 12.5 μm centre−centre spacing)coupled to a 12-bit ADC, which connects to a PC via a serial RS232interface. The quartz fiber 33 transmits the light to the entrance slit41 (width 50 μm), which guarantees a spectral resolution of 0.7 nm. Thewavelength region observed by the detector was set from 245 nm to 380nm. The wavelength to pixel mapping as well as the instrumental lineshape was determined by taking a spectrum of a low-pressure mercuryemission lamp. The entire system (notebook PC, spectrometer and steppermotor) operates on 12 V for about 24 hours from a standard car battery.The known Differential Optical Absorption Spectroscopy (DOAS) techniquewas applied to identify and quantify the atmospheric trace gases bytheir specific narrow band absorption features in the UV and visiblespectral regions.

The fundamental equation of absorption spectroscopy is theBeer-Lambert's Law. It describes the decrease of light intensity whilelight passes matter.

I(λ,L)=I ₀(λ)*e ^(−S*σ)

I₀ (λ)—light intensity outside the gas layer to be measuredI (λ, L)—light intensity after passing the gas layer to be measuredλ—wavelength of the radiationc (l)—concentration of the trace gas as function of the position l alongthe light pathσ(λ, T)—trace gas absorption cross sectionT—temperatureL—length of light path

S = ∫₀^(L)c(l) l( = c * L,    if  c = const.) − Definition  of  slant  column  density  (S C D)

SCD were directly determined here as results of the measurements. Fluxescan however be calculated using further estimates. Automatic dataacquisition, real time spectral evaluation and flux calculations wasperformed by a home-made C-program running on a notebook computer underthe WINDOWS operating system.

FIG. 11 shows some results from the testing of the inventive measuringdevice 20. For comparison, tests were also made with a conventionalinstrument, using a flat vertical scanning plane 12 passing zenith,operated in parallel from the same location. FIG. 11 shows the resultsfrom the different devices in the form of a representation of SO₂ in aparticular air column/FOV-direction 14, 24 as a function of scan stepangle α as given by the extent of rotation of the motor axis 35. Notethat in contrast to FIGS. 3 and 4, the x-axis of FIG. 11 starts at 0°.Thus only “half” scans was performed, starting at zenith going down toone horizon. Over a time period of 76 minutes 22 simultaneous scans wasperformed and the averaged results are shown. The averaged resultsobtained with the inventive measuring device 20 are given by solid lines120 and the results obtained with the conventional equipment are givenby dashed lines 110. The approximate centre position of each peak isindicated by arrows. At the time of the experiment the wind directionwas such that the volcano plume passed the measuring device at somedistance at the side, in similarity with plume P3 in FIGS. 1 and 2. FIG.11 clearly shows that the peak recorded with the inventive measuringdevice is positioned sufficiently far away from the end of the scanallowing a reliable measurement. In contrast, the peak recorded with theconventional equipment is too close to the end of the scan to allow fora reliable measurement.

The present invention is not limited by the embodiments described abovebut can be modified in various ways within the scope of the claims. Forinstance, a variation of the direction of the field-of-view (FOV) can beachieved by a computer controlled variation of the position of thetelescopic member 30. Alternatively, a computer controlled mirror couldbe used instead of mounting the mirror 32 to a rotatable motor axis 35.

Moreover, the spectrometer 31 may be replaced by another light analysisunit by which the slant column density of a gas or aerosol may bedetermined. In some applications it may for instance be sufficient to beable to detect a certain, narrow wavelength interval.

Further, a mirror could be used instead of the lens 39 as light focusingmember, and a prism could be used instead of the mirror 32 as lightrefracting member. The optical fiber is optional; the spectrometer 31 orother light analysis unit may be connected directly to the telescopicmember 30.

It should also be noted that it is not necessary that the measurement iscarried out stepwise; the scan angle α can be varied more or lesscontinuously as to let the FOV 24 continuously scan the scanning layer22.

In a preferred variant of the inventive device the mirror 32 isadjustable such that the angle β becomes adjustable.

The angles can vary within their defined ranges; −180°≦α≦180°, 0°≦β<90°,−90°≦δ≦90°. Although the inclination angle δ will be ≧0° in mostapplications, i.e. the cone is either horizontal or has its wider partpointing upwards as to point towards a source positioned at a higherlevel, it is possible to use a negative δ, i.e. where the cone has itswider part pointing downwards. It may also be noted that the horizontalpointing direction of the cone, i.e. the direction of the symmetry axisof the cone as projected onto the horizontal plane if the cone isinclined relative to this plane, does not necessarily have to beidentical to the horizontal direction between instrument and source. Forinstance, if the view towards the source for topographical reasons isobstructed, one may “aim” with the cone at the side of the source.

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present invention as defined by the appended claims.

1. A method for measuring emissions of gaseous substances to theatmosphere using scattered sunlight spectroscopy and an opticalmeasuring device comprising a telescopic member defining a field-of-viewof said optical measuring device, and a scanner for controlling thevariation of the direction of said field-of-view to scan a predeterminedlayer of the atmosphere, said method comprising scanning saidfield-of-view to scan said predetermined layer of the atmosphere in theform of at least a part of a cone having its apex positioned at saidoptical measuring device and having a cone angle β.
 2. The methodaccording to claim 1, wherein said cone angle β is up to 80°.
 3. Themethod according to claim 1, wherein said cone angle β is at least 20°.4. A method according to claim 1, wherein said cone angle β is from 40to 70°.
 5. A method according to claim 1, wherein said cone has its axisof symmetry in a horizontal direction.
 6. A method according to claim 1,wherein said cone has an axis of symmetry exhibiting an inclinationangle (δ) relative to a horizontal plane.
 7. A method according to claim6, wherein said inclination angle (δ) is 90°−β.
 8. An optical measuringdevice for measuring emissions of gaseous substances to the atmosphereusing scattered sunlight spectroscopy, said device comprising atelescopic member defining a field-of-view of said measuring device, anda scanner for controlling the variation of the direction of saidfield-of-view to scan a predetermined layer of the atmosphere, saidscanner being adapted to scan said predetermined layer of the atmospherein the form of at least a part of a cone having its apex positioned atsaid optical measuring device and having a cone angle β.
 9. The deviceaccording to claim 8, wherein said cone angle β is up to 80°.
 10. Thedevice according to claim 8, wherein said cone angle β is at least 20°.11. The device according to claim 8, wherein said cone angle β is from40 to 70°.
 12. The device according to claim 8, wherein said scannercomprises a light refracting member arranged to deflect saidfield-of-view by directing a portion of incoming sunlight towards saidtelescopic member.
 13. The device according to claim 12, wherein saidlight refracting member is arranged in such a way that the non-deflectedfield-of-view and the deflected field-of-view form the angle β.
 14. Thedevice according to claim 12, wherein said light refracting member isrotatably arranged in relation to said telescopic member wherebyrotation of said light refracting member allows the field-of-view toscan said predetermined layer of the atmosphere.
 15. The deviceaccording to claim 14, including a motor having a motor axis, andwherein said light refracting member is arranged on said motor axisconnected to said motor.
 16. The device according to claim 12, whereinsaid light refracting member is a mirror.
 17. The device according toclaim 12, wherein said light refracting member is a prism.
 18. Thedevice according to claim 8, including a light analysis unit opticallyconnected to said telescopic member.
 19. A device according to claim 18,wherein said light analysis unit comprises a spectrometer.
 20. Thedevice according to claim 8, wherein said telescopic member comprises alight focusing member adapted to focus light that enters said telescopicmember and an opening adapted to direct the focused light out from saidtelescopic member.
 21. The device according to claim 20, wherein saidlight focusing member is a lens.
 22. The device according to claim 20,wherein said light focusing member is a mirror.